Resources for Blender
Every section listed alphabetically

Textures


CC0Textures CC0/Public domain license
CG Textures License covers commercial use, but not open source software. *Registration required, free at low resolutions*
Good Textures Similar to CG textures, but with a more lenient license. *Registration required*
Lovetextures
Mayang's Textures
nordicfx non-commercial
OpenGameArt CC-BY
Pixar 128 Library 128 textures with normal and height maps originally made by Pixar Studio in 1993 and updated in 2015, CC BY 4.0
PixPlant's Seed Images Various licenses
Plain Textures *Registration required*
Poliigon Paid registration required for high resolution versions; several high resolution free textures available
Share Textures Collection of CC0 textures
Subtle Patterns
Textures.TheFree3DModels Attribution required Noncommercial
Texturebits
TextureHaven CC0
Texture lib free for commercial and none commercial use, license is very lenient
Texture Mate
Texture Ninja A collection of CC0 textures 
texture pilot *Registration required*
Texture X Free high resolution textures.
Yethiel's Collection of Textures Various textures collected by an individual with CC0 license
3dmd.net  Small library and no high resolution
3DTextures




HDR/JPEG Skydome Images


HDRI Haven CC0 
Adaptive Samples CC-BY-SA.
Chocofur HDRs High quality paid HDRs; one free available here
GiantCowFilms.com CC0/Public Domain
HDRI Hub Free Samples
Maxwell render resources
MrBluesummers HDR lights.
NoEmotion HDRs High resolution HDRs; CC-BY-ND
nordicfx non-commercial
Open Footage CC-BY
Philo's Sky Collection
sIBL hdrlabs CC-BY-SA
 0ptikz's skies




Stock Photos


BucketListly CC-BY-NC
Foter Various CC Licenses
FreeImages
Image After
Immediate Entourage many masked images
Library of Congress Public Domain
MorgueFile
openclipart
OpenImages
Pexels CC0
Pixabay free, but with some restrictions
Pond5 *Registration required*
PhotoPin Various CC Licenses
PublicDomainPictures reduced resolution free, Public Domain
Stockvault non-commercial
Rawpixel free for commercial
Unsplash CC0
USDA Public Domain
US Navy Public Domain
Visible earth NASA Public Domain
3dtotal some textures too
Pxhere Free stock photos